With Newt Gingrich as their common target, the Republican presidential hopefuls piled on the new party front-runner in a lively debate Saturday night, jabbing him over his political consistency, the sturdiness of his character and the plausibility of his policy proposals.

One after another, rivals portrayed the former House speaker — who looked on stern-faced — as an opportunist who changes his beliefs to suit the political times and his personal ambitions.

"He's been on different positions, you know, on so many issues," said Rep. Ron Paul of Texas, voicing a theme picked up by others on the Drake University stage who each insisted he or she alone was the true conservative who could best take the fight to President Obama.

Gingrich’s history of marital infidelity arose when the co-moderator, George Stephanopoulos of ABC News, asked whether voters ought to consider whether a candidate has been faithful to their spouse when deciding whom to support for president.

Texas Gov. Rick Perry, who is avidly courting Iowa’s social conservatives, responded firmly, saying that a politician who will cheat on his wife cannot be trusted.

"If you will cheat on your wife, if you will cheat on your spouse, then why wouldn’t you cheat on your business partner, or why wouldn’t you cheat on anyone for that matter?" Perry asked.

Gingrich, who is married to his third wife, Callista — with whom he conducted an extramarital affair — seemed ready for the question and agreed that voters ought to consider the matter. “I think people have to look at a person to who they are going to loan the presidency and they have a right to ask every single question,” he said.

Gingrich reiterated his prior statements that he had made mistakes and sought forgiveness and that people ought to consider who he is now: “I'm also a 68-year-old grandfather, and I think people have to measure who I am now and whether I am a person they can trust.”

The candidates initially seemed reluctant to engage. But once prodded by Stephanopolous, they quickly jumped in

Mitt Romney, who has been running at or near the front of polls until Gingrich's recent surge, challenged Gingrich’s call to colonize the moon and to change child labor laws so inner-city students can go to work cleaning their classrooms. Romney contrasted his background in the business world with Gingrich's long career in Washington.

"Let's be candid," Gingrich shot back. “The only reason you didn't become a career politician is you lost to Teddy Kennedy in 1994.”

The rejoinder — a reference to Romney's unsuccessful U.S. Senate bid before winning the Massachusetts governorship eight years later — was the closest Gingrich came to showing pique.

Acting like the college lecturer he once was, he rebutted his opponents point by point, saying, for instance, that colonizing the moon would inspire students to study math and science and that putting poor students to work was a way to instill a healthy work ethic.

He also glossed over some facts, ignoring his past support for cap-and-trade legislation to fight global warming, and sidestepped others, defending his acceptance of more than $1.6 million from Freddie Mac — the federal insurer he has attacked for causing the housing crisis — by saying it was the sort of private sector work that Romney was extolling.

“K Street's not the private sector,” a laughing Romney responded, referring to the corridor housing the Beltway's major lobbying firms.

Romney, who has struggled to shuck his patrician image, probably did not help his cause with an impromptu proposition made Saturday night.

Perry said that Romney had written in his campaign book that his healthcare mandate “should be a model for the country,” a claim Romney has made in the past but not in the pages of his text.

In response, Romney stuck out his hand and offered to bet Perry $10,000 that he was wrong -- a gesture that is unlikely to play well amongst Iowans, who tend to recoil from ostentatious displays of wealth.

The nearly two-hour session was the 12th of the nominating season and the first since Gingrich rose from political near-death to become the latest candidate to lead the pack, a status confirmed by his front-and-center positioning on stage and the proverbial bull's-eye on his back.

Rep. Michele Bachmann, who enjoyed a brief stint at the top of Iowa polls before falling back, cited Gingrich's past support for a mandate requiring individuals to purchase health insurance, a provision of Obama's healthcare plan that conservatives revile. She noted Romney's Massachusetts legislation had a similar provision and, lumping two together, suggested to voters, “If you want a difference, Michele Bachmann is the proven conservative, not Newt Romney.Ö”

Gingrich defended his support for the mandate, which he now opposes, by saying it was the best way at the time to fight the healthcare overhaul plan put forth in the 1990s under President Clinton. But former Pennsylvania Sen. Rick Santorum echoed those questioning Gingrich's consistency, saying he came out against the mandate even then.

The debate came at an important juncture in the turbulent race, with the exit of erstwhile poll leader Herman Cain, the surge of Gingrich and, not least, the impending arrival of Christmas and New Year's, which are competing for voters' attention. (The wall-to-wall TV advertising is unlikely to offer much in the way of good cheer.) Iowa's precinct caucuses, the first step in the presidential selection process, will be held Jan. 3.

The candidates will meet again Thursday night at a second Iowa debate, scheduled for Sioux City.